1. Field of the Invention
The present invention relates to a clamping apparatus adapted to operationally press an object to be clamped or fixed (referred to as a clamped object hereinafter) such as a mold, a work pallet and the like onto a fixed angular table of a processing machine such as an injection molding machine, a machining center and so on, and more particularly to a fluid clamping apparatus with a clamp locking means which operates to prevent the clamped object from being unclamped by an external force when a clamping fluid pressure abnormally lowers.
2. Description of Prior Art
Such a clamping apparatus is disclosed in Japanese Provisional Patent Publication No. 1-154833.
As shown in FIG. 13, this is adapted to advance a first piston 312 from an unclamping position Y (a figure depicted by an alternate long and two short dashes line) to a clamping position X (a figure depicted by a solid line) by means of a fluid pressure within a first actuation chamber 313 of a clamping first cylinder 309, and a locking chamber 326 is communicated crosswise with the first actuation chamber 313 with a locking wedge 327 inserted into the locking chamber 326.
This wedge 327 is adapted to be so moved as to be changed over between a locking position M (a figure depicted by a solid line) for advancement to the first actuation chamber 313 and an unlocking position N (a figure depicted by an alternate long and two short dashes line) for retraction into the locking chamber 326 by means of a fluid pressure within a second actuation chamber 393 of a second cylinder 329 through a second piston 339. A third actuation chamber 340 is formed below the second piston 339, and a locking push spring 395 is mounted within the second actuation chamber 393.
When a clamping arm 307 is changed over from an unclamping condition illustrated by a figure depicted by an alternate long and two short dashes line to a clamping condition illustrated by a figure depicted by a solid line, a pressurized fluid is discharged from the third actuation chamber 340 and the pressurized fluid is supplied to both the first actuation chamber 313 and the second actuation chamber 393. Thereupon, the first piston 312 is advanced leftwards from the unclamping position Y so as to swing a clamping arm 307 toward the clamped object 302. Simultaneously, the wedge 327 is advanced from the unlocking position N to the locking position M by means of the fluid pressure within the second actuation chamber 393 and the resilient force of the push spring 395, so that its wedge surface 348 can be engaged wedgewise with a wedge receiving surface 349 of the first piston 312. Subsequently, the first piston 312 is pushed strongly to the clamping position X by means of a resultant force obtained from both a wedgy engaging force of the wedge 327 advanced subsequently thereto and a fluid pressure within the first actuation chamber 313.
In addition to a problem that a height of a fluid clamp becomes tall because the second cylinder 329 projects upwardly from the first cylinder 309, there is also the following problems associated with the above-mentioned conventional construction.
That is, in the above-mentioned wedgy engagement type fluid clamp, the wedge 327 is adapted to be actuated for locking to the locking position M against a dynamical friction force acting between both surfaces of the wedge surface 348 and the wedge receiving surface 349 at an end stage of clamping actuation. To the contrary, at an initial stage of unclamping actuation it is necessary to unlockingly actuate the wedge 327 to the unlocking position N against a statical friction force which is remarkably larger than the dynamical friction force. It is the reason why a coefficient of statical friction is remarkably larger than a coefficient of dynamical friction and a metal contact is caused because a lubricating oil between both those surfaces 348, 349 would have been squeezed out by an excessively large surface pressure at the end stage of clamping actuation.
Further, since a resultant force obtained from the fluid pressure within the second actuation chamber 393 and the resilient force of the locking push spring 395 is utilized at the time of locking actuation of the wedge 327, the locking push force becomes large. Therefore, when the wedge 327 is surely actuated for unlocking, it is necessary to enlarge a cross-sectional area of the second actuation chamber 340 by making the second piston 339 having a large diameter. Then, it is necessary to set an inclination angle .alpha. of the wedge surface 348 to such a value as being small as possible in order to surely actuate the wedge 327 for unlocking. Therefore, a lower portion of the first piston 312 projects backwardly.
Accordingly, the fluid clamp becomes larger in size because of its taller height, its second piston 339 having a larger diameter and its longer first piston 312.
There are also the following problems.
Within a duration from an initial stage of the clamping actuation illustrated in a figure depicted by the alternate long and two short dashes line to a beginning of the wedgy engagement, the lower surface of the wedge 327 is brought into strong contact with the outer surrounding surface of the first piston 312 by means of the resultant force obtained from the fluid pressure within the locking second actuation chamber 393 and the resilient force of the locking push spring 395. Therefore, the outer surrounding surface is apt to be damaged and thus to cause an oil leakage from the first actuation chamber 313. This problem may come out as a conspicuous evil influence when a clamping thickness of the clamped object such as the mold 302 would become large. That is, it is the reason why when the clamping thickness would become large, a load for the first piston 312 may become large at a stage of a small swinging angle of the clamp arm 307 and also an interferential degree between the outer surrounding surface of the first piston 312 and the lower surface of the wedge 327 may become large.